Significant improvements have been made in organic light-emitting diode (OLED) technology, including improved efficiency, high brightness, and low drive voltage. Advances in the field have led to the realization of high-efficiency full-color and white-color OLEDs. Despite advances, operational stability remains a challenge for OLED technology. One avenue for producing higher efficiency devices utilizes low work-function metals (such as Ca or Ba), which are useful as cathode materials to facilitate electron injection. However, low work-function metals are very sensitive to moisture and oxygen and operation often causes the formation of quenching sites at areas near the interface between the electroluminescent layer (EL) and cathode. Additionally, metal ions formed at the interface tend to migrate into the EL layer, thus affecting the long-term stability of these devices.
One solution to these problems is the use of high work-function metals (such as Al, Ag, or Au) as cathode materials because of improved environmental stability and increased simplicity fabricating devices. While high work-function metals have improved stability, they suffer from poor electron injection into the EL material. Improved electron injection from high work-function metals has been attempted by inserting a thin layer of polar or ionic insulating species, such as lithium fluoride (LiF) or cesium fluoride (CsF). However, these methods exhibit cathode-material dependence and they are not universally applicable to other high work-function metals such as Ag or Au. There still remains a need for an efficient electron injection material that is compatible with high work-function cathodes but does not degrade device performance during operation.